My questions are, is this likely to get dropped?
No. Prosecutors get paid to make convictions not to drop charges.
we had no idea we were breaking the law.
Ignorance of the law is not a defense and the AZ statute does provide for this:
A.R.S. §13-1502, §13-1503 and §13-1504, "Trespass" is generally when a person knowingly enters or remains unlawfully on a piece of property after they have been requested to leave, or without the expressed permission of the owner, or in violation of a posted sign warning of Trespass. (emphasis mine)
A.R.S. §13-1503, a Criminal Trespass in the "Second-Degree" occurs when a person unlawfully enters or remains on a non-residential structure or fenced commercial yard.
You didn't have permission from the school to be there and there is a posted sign (even if you didn't see it) that warns being there is trespassing.
Am I going to most likely have to pay like a $500 fine?
Worse. Criminal Trespass in the "Second-Degree" can be charged as a class two (2) misdemeanor. A class two (2) misdemeanor carries a range of punishment of probation and up to four (4) months in jail. Additionally, a fine of $750.00 plus 80% surcharges can be imposed.
Hire a lawyer. The potential for a criminal conviction that could create long term problems in your life exists. This is NOT a DIY project nor is it likely to just "go away" and be dropped.
"That's just my opinion, then again I might be wrong." Dennis Miller